1. Field of the Invention
The present invention relates to a device for determining the running speed of a vehicle, on the basis of a pulse signal generated from a vehicle speed sensor.
2. Discussion of the Related Art
In the art of controlling the shifting of an automatic transmission of a motor vehicle, there has been used a device for calculating the running speed of the vehicle, from pulses which are generated by a speed sensor in response to the rotation of a rotating member whose speed changes with the speed of drive wheels of the vehicle. More specifically, since the period of the pulses changes with the actual vehicle speed, the actual vehicle speed can be calculated from time to time, on the basis of the detected periods of the successive pulses. In this type of known vehicle speed calculating device, a difference between the periods of the received two successive pulses may exceed a given limit, in the event of a trouble with the device, for example, electrical failure such as discontinuity of signal lines or poor contacts of connectors between the sensor and the device. If one or more pulses were missing due to such electrical failure during a time length between the actually generated and received two successive pulses, the period of the last received pulse (currently detected pulse) would differ from that of the preceding pulse (pulse received in the preceding calculation cycle), by an abnormally large amount, namely, by a difference larger than a given threshold. In this abnormal case, the last received pulse is ignored, and the period of the preceding pulse is stored as the period of the last received pulse. The vehicle speed is calculated from the stored periods of the successive pulses, which include the last pulse whose period is replaced by that of the preceding pulse in the abnormal case indicated above. Examples of such vehicle speed calculating device are disclosed in laid-open Publications JP-A-59-210369, JP-A-59-210370, JP-A-59-210371 and JP-A-59-210374 (publications of unexamined Japanese Patent Applications) and U.S. Pat. No. 4,718,013.
As indicated above, the vehicle speed is calculated on the basis of a predetermined number of stored successive pulses including the last pulse. That is, a moving average of the stored periods of the predetermined number of the successive pulses is used to calculate the vehicle speed. Accordingly, the vehicle speed calculated on the basis of the above average will not be zeroed or almost zeroed as soon as the actual vehicle speed has been zeroed. Consequently, the automatic transmission which is shifted depending upon the calculated vehicle speed may not be sufficiently shifted down when the vehicle is suddenly stopped with abrupt brake application, for instance. In this case, the stopped vehicle cannot be re-started with a sufficient drive force being transmitted to the drive wheels from the transmission.
For improving the known vehicle speed calculating device, it is considered to provide the device with means for determining that the vehicle speed is zero, if a pulse has not been received from the vehicle speed sensor, for a time duration exceeding a predetermined threshold. This arrangement permits the transmission to be sufficiently shifted down upon stopping of the vehicle with abrupt brake application. However, the same arrangement causes the determination of zeroing of the vehicle speed, even in the event that the device does not receive pulses from the sensor, due to poor contact of the connector or discontinuity of the signal lines, for example, while the vehicle is in fact running at a certain speed well above the zero level. In this case, the transmission may be unexpectedly and undesirably shifted down even though the vehicle is running at a relatively high speed.